Simple Long Duration Timer Circuit using Two Transistors

Interested in creating a timer that can switch an object on or off after two or three hours?

Using only two transistors this article teaches how to create a basic timer circuit.

For anyone interested in learning more about electronics as well as creating something practical this project is fantastic.

What is a Long Duration Timer:

An electrical circuit designed to carry out a timing work over an extended period of time is known as a long duration timer.

Depending on what the application requires this kind of timer might create delays that may last from a few minutes to several hours or even days.

Long duration timers are usually found in many different kinds of electronic systems and applications that require a time delay for specific tasks.

Circuit Working:

Parts List:

The simple long duration timer circuit shown in the diagram above operates as follows:

Just by pressing the push button starts the charging of the 1000uF capacitor.

Even when the push button is released the NPN BC547 transistor is triggered and remains in its current state.

The 1000uF capacitor slowly discharges through the 2.2M resistor and the NPN emitter.

Activation of BC547 and BC557:

• When the BC547 is triggered it activates the PNP BC557 transistor.
• BC557 in turn triggers the relay and the connected load.

Persistent Operation:

• The circuit continues its operation as long as the charge in the 1000uF capacitor does not fall below the BC547 cutoff threshold values.

The circuit becomes a very accurate long duration timer when the 1k and 1N4148 networks are added.

By connecting the resistor and diode this network makes sure that the remaining charge is forced to discharge totally through the relay coil when the transistors break the latch if the capacitor is not sufficiently charged.

By checking that the capacitor is completely low this feature gets the circuit ready for a new cycle.

Formulas and Calculations:

The below mentioned formula calculates the charge time (t_charge) of a capacitor in a simple RC charging circuit.

Let us understand the formula and calculation for the specific values from the above circuit diagram R = 2.2M and C = 1000uF:

t_charge = R × C

where,

• t_charge represents the time it takes for the capacitor to charge measured in seconds (s).
• R represents the resistance in the circuit measured in ohms Ω in this article R is 2.2 MΩ (mega ohms)
• C represents the capacitance measured in farads F here C is equal to 1000 microfarads (uF).

Calculations:

Enter the numbers in the formula:

t_charge = 2.2MΩ × 1000uF

Optional Unit Conversion:

1: Mega ohm farads are not commonly used as a unit of measurement despite the fact that they are potentially multiplied by microfarads.

Converting one of the units is useful to get a clearer reaction.

Since a megaohm contains one million ohms we may divide MΩ by that number to get F.

t_charge = 2.2MΩ × (1000uF / 1,000,000)

t_charge = 2.2Ω × 0.001F

Reduce the complexity of the equation:

t_charge = 0.0022F (Since 2.2 x 0.001 = 0.0022)

In this circuit the capacitor will take about 2.2 seconds to charge.

Note:

In this calculation it is first assumed that the capacitor has completely run out (at 0 volts).

The formula simply calculates the time it will take for the voltage of the capacitor to increase to about 63% of its final value which is the voltage source of the circuit.

A fully charged capacitor can possibly take a limitless amount of time to reach 100% voltage however 63% is usually believed to be adequate charging.

Resistor tolerance and capacitor leakage current are two examples of the variables that can affect the actual charging time.

2: To estimate the capacitors discharge time (tdischarge) the formula below can be applied to a simple RC discharging circuit.

Let us break down the formula and calculation for the specific values from the above circuit diagram R = 2.2M and C = 1000uF:

t_discharge = R × C

A More Precise Method:

The following formula is used to calculate discharge time more accurately particularly when working with a certain beginning voltage:

t = -RC * ln(V_discharged / V_initial)

where,

• The discharge tie t is expressed in seconds (s).
• R is the circuits resistance expressed in ohms Ω.
• The capacitance expressed in farads F is represents by C.
• The voltage you want the capacitor to discharge to which is 0V in most applications is represented by the variable V_discharged.
• The term V_initial refers to the capacitors initial voltage prior to the start of discharge.
• The natural logarithm function or ln is shown on the majority of scientific calculators.

Calculation:

Consider that the capacitor in the circuit has a 5V Vinitial initial voltage. You want to know how long it takes for it to discharge to 1V (Vdischarged).

The values of the circuits will be:

C = 1000uF and R = 2.2MΩ.

Note: To use the ln function to make sure your calculator is in radian mode.

Compute the outcome:

The resulting value will be negative because time cannot be negative.

We may ignore the negative sign because we are just interested in the duration.

Approximately 0.0008 seconds, or 0.8 milliseconds, will be the outcome.

Remember:

This is the approximate discharge time needed to reach 1V.

The discharge slows down somewhat as the voltage drops but the actual time to reach 0V would be greater.

Use circuit simulation software or refer to the datasheets for particular capacitors which may include discharge time curves for greater accuracy.

How to Build:

Follow the below steps for building a Simple Long Duration Timer Circuit using Two Transistors:

Connect BC547

• Connect the push button to the base of the BC547 transistor.
• Connect the collector of BC547 to the positive terminal of the 1000uF capacitor.
• Connect the emitter of BC547 to the 2.2M resistor and the other end of the resistor to the negative terminal of the capacitor.

Activate BC557 and Relay:

• Connect the collector of BC547 to the base of BC557.
• Connect the emitter of BC557 to the relay coil, and the other end of the coil to the positive supply.
• Connect the collector of BC557 to the load and the negative supply.

Integrate resistor and diode:

• Connect the 1k resistor and the 1N4148 diode in series to the collector of BC547 and the positive terminal of the 1000uF capacitor.

Conclusion:

You may build a very accurate and dependable long duration timer circuit by following this method.

By using the above formulas one may change the timers duration according to the values of the resistor and capacitor.

Enjoy the timer you made yourself!

References:

Timer circuits – difficulty analyzing